Rule: Such is a determiner and modifies nouns; so is an adverb and modifies adjectives and adverbs.
Examples of errors:
So and that can be used to modify adjectives and adverbs in sentences like (1) or:
There were 2000 people in the theater. I didn't expect it to be so/that full
I played terribly yesterday, but I'm not playing so/that badly today.
These are equivalent to:
I didn't expect it to be as full as it was yesterday.
I'm not playing as badly today as I did yesterday.
Such is often used in a similar way, provided it modifies a noun rather than an adjective or adverb (as in (1)):
I didn't expect to play so/that good a game.
= I didn't expect to play such a good game.
In the second example, such modifies game, whereas in the first example so and that modify good. Similarly, the comparative construction so that) is used to modify adjectives or adverbs, and such that) to modify nouns:
He's so naive (that) he'd believe anything.
He's such a naive person (that) he'd believe anything.
These constructions can often be related semantically to enough and too to indicate 'sufficiency' and 'excess', respectively (cf. 7):
He's naive enough to believe anything ('sufficiency')
He's too naive (for me) to trust ('excess')